1. Field of the Invention
The present invention relates to a circuit design, and more particularly, to a design methodology of a multi-input operational amplifier.
2. Description of the Related Art
A multi-input operational amplifier is commonly seen in meter amplifier fields. Generally, a multi-input operational amplifier includes multiple differential input terminals connected to transconductors, and then connected to a primary amplifier. FIG. 1 shows a structural diagram of a well-known two-input operational amplifier. The first differential input pair 102 and 104 feed separately into non-inverting (+) and inverting (−) ends of the first transconductor 110. The second differential input pair 106 and 108 feed separately into non-inverting (+) and inverting (−) ends of the second transconductor 112. The two transconductors are connected through an adder 114, and then output through a primary amplifier 116. The primary purpose of the operational amplifier with differential input ends is to avoid common mode noise or interference.
However, one input pair of a general multi-input operational amplifier affects the other input pair, and causes an input voltage offset to the other input pair. FIG. 2 shows an input/output curve offset phenomenon of the second input terminal resulting from the level shift of the first input terminal. The curves 22, 24, 26 in FIG. 2 represent, respectively, the relations of the inverting input and output terminals of the second input pair under the conditions where the voltage of the inverting input of the first input pair is equal to that of the non-inverting input terminal, the voltage of the inverting input of the first input pair is slightly smaller than that of the non-inverting input terminal, and the voltage of the inverting input terminal is equal to zero voltage. The difference between the transition point of the curve 22 and that of the curves 24 and 26 represents that the first input terminal affects the second output terminal.